The present invention refers to a support device for a centrifugal separator having a centrifuge rotor, which is provided in a frame member to be rotatable about an axis of rotation by means of at least one bearing member having a middle plane, wherein the axis of rotation is substantially perpendicular to the middle plane of the bearing member, wherein the support device is arranged to be provided between the bearing member and the frame member and to permit relative movements between the centrifuge rotor and the frame member, and comprises two first substantially stiff portions which are substantially immovable in relation to the bearing member, two intermediate elastic portions and two second substantially stiff portions which are substantially immovable in relation to the frame member, and wherein the two first stiff portions seen in an axial section adjoin a respective intermediate portion along a first borderline and the two second stiff portions seen in said axial section adjoin a respective intermediate portion along a second borderline.
Such support devices of centrifugal separators, and in particular for the so-called neck bearing of the centrifugal separator, are to present a certain resiliency, i.e. to permit radial and axial relative movements between the frame member and the centrifuge rotor. Thereby, it is important that the support device has a stiffness, which is low enough to maintain the forces acting on the neck bearing on a low level. Large forces acting on the neck bearing reduce the lifetime thereof. However, the support device may not be too weak since the critical number of revolutions of the centrifuge rotor in that case will be too low, which results in a risk for instability and large oscillating movements. Thus, the support device is to present a sufficient but not too high stiffness in a radial as well as an axial direction. In addition to permit such relative movements, the support device also is to have a certain damping effect in the radial as well as the axial direction.
It is known to provide such a resiliency and damping by means of a number of steel spring buffers, which are provided around the neck bearing. Such steel spring buffers comprise a helical spring by which a desired elasticity or resiliency may be provided. By permitting the spring buffers to slide against a surface connected to the bearing member or the frame member, a certain damping may be obtained. However, by such spring buffers the support device becomes complicated as a whole and requires a relatively extensive mounting work.
Furthermore, it is known to utilize rubber elements extending as a ring about the whole rotor spindle or being provided as separate rubber elements about the rotor spindle in order to support the centrifuge rotor. The rubber elements existing today are arranged to permit shearing of the rubber as a result of radial relative movements. The spring constant of the rubber with respect to compression forces is, however, relatively large whereas the spring constant with respect to shearing forces is relatively small. Consequently, such rubber elements have a relatively high axial stiffness and a relatively low radial stiffness. If, in such constructions, one tries to increase the stiffness by choosing a harder rubber (higher shore number), problems frequently occur since the quality of a rubber which is sufficiently hard is unsatisfactory, i.e. the through hardness of the rubber element available varies and in addition such hard rubber ages faster.
SE-B-137 768 discloses a centrifugal separator having a centrifuge rotor, which is provided in a frame member to be rotatable about an axis of rotation by means of a bearing member. The known separator comprises a support device, which is provided between the bearing member and the frame member, and permits relative movements between the centrifuge rotor and the frame member. The support device comprises a radially inner stiff portion, an intermediate elastic portion of rubber and a radially outer stiff portion, wherein the intermediate portion is arranged to receive radial relative movements by a pure compressing of the rubber. The support device also comprises an axially upper stiff portion, an axially intermediate elastic portion of rubber and an axially lower stiff portion wherein the intermediate portion is arranged to receive radial relative movements by a substantially pure shearing of the rubber.
FR-A-884 635 discloses different designs of a support device for a neck bearing for a centrifuge rotor of a centrifugal separator. According to one embodiment, the support device comprises an annular rubber element which has an L-shape seen in an axial section and which thus extends radially outwardly and axially downwardly from the bearing to permit and damp axial as well as radial movements, i.e. tilting or pivoting movements.
DE-B-1 069 431 discloses a support device for a centrifuge rotor. According to the embodiment disclosed, the support device comprises two intermediate elastic portions. The support device is displaced along the axis of rotation with respect to the bearing.
FR-A-1 263 432 discloses another support device for a centrifuge rotor. The device comprises two intermediate elastic portions forming borderlines to the respective stiff portions. The borderlines extend in a radial plane with respect to the axis of rotation.
The object of the present invention is to provide a constructively simple support device, which is arranged to permit relative movements between the centrifuge rotor and the frame member, and to provide damping of these movements. In particular, the support device is to be designed in such a manner that a desired spring constant and a desired damping may be obtained in a simple manner in axial as well as radial direction.
This object is obtained by the support device initially defined, which is characterized in that at least one of said second borderlines slopes in relation to the axis of rotation, that the extensions of the second borderlines, which extensions are directed outwardly from the axis of rotation, intersect, and that the two intermediate portions are provided on a respective side of the middle plane. By such an elastic or resilient portion, which is arranged in such a manner that an outer borderline slopes in relation to the axis of rotation, i.e. has an angle to the axis of rotation which is larger than 0xc2x0 and less than 90xc2x0, the deformation arising in the elastic portion will comprise both a compressing and a shearing deformation of the elastic element. By choosing the angle of inclination by which said second borderline slopes, the degree of compressing and shearing, respectively, may thus be adapted to the individual construction in such a manner that a suitable spring constant and stiffness are obtained. By such an elastic portion, it is also possible to obtain a low stiffness against tilting, although the radial stiffness is relatively high. This reduces the loads onto the bearing member and prevents breaking between the inner ring and the outer ring of a roller bearing. Furthermore, such a frame member may be manufactured and mounted in a simple manner and with small dimensions, i.e. the frame member according to the invention requires a relatively small space.
According to an embodiment of the invention, the radially inner end of the first borderline of at least one of the intermediate elastic portions is located radially inside the radially inner end of the second borderline of this intermediate elastic portion. Furthermore, the radially outer end of the first borderline of at least one of the intermediate portions may by located radially inside the radially outer end of the second borderline of this intermediate elastic portion.
According to a further embodiment of the invention, at least one of said first borderlines slopes in relation to the axis of rotation and the extensions of the first borderlines, which extensions are directed outwardly from the axis of rotation, intersect. According to this embodiment, both the second, radially outer borderline and the first, radially inner borderline may slope in relation to the axis of rotation. In such a manner, the degree of compressing and shearing, respectively, may in a very simple and secure manner be determined by a suitable choice of a common angle of inclination or different angles of inclination of both the borderlines. Furthermore, by such an embodiment, a low stiffness against tilting and a high stiffness against radially movements may be obtained in a simple and secure manner.
According to an advantageous embodiment of the invention, the two intermediate elastic portions are arranged to extend about said axis of rotation. Furthermore, also the two first stiff portions may be arranged to extend about said axis of rotation as well as the two second stiff portions.
According to a further embodiment of the invention, each intermediate elastic portion is formed by a ring of a rubber-like material. By such an elastic or resilient portion of rubber, the support device may be realized in a simple manner.
According to a further embodiment of the invention, the two first borderlines are arranged to slope in relation to said axis of rotation by a respective angle of inclination. The absolute values of these angles of inclination may be different or substantially equal. Furthermore, the two second borderlines may be arranged to slope in relation to the axis of rotation by a respective angle of inclination. Also, the absolute values of the angle of inclination of the second borderlines may be different or substantially equal.